In our Mac Geek Gab forums, Brian Monroe has a warning for people wanting to try the macOS Big Sur beta.
APFS
He found that you can’t install the beta on the same APFS partition you used for previous versions of macOS. If you do, you’ll see a warning like this:
You’ll also see an error about an incompatible disk:
People on the linked Reddit thread mentioned it could depend on whether you install the beta on a separate volume on a partition.
Currently we’re on the first developer beta, so bugs like this are to be expected (well, maybe not bugs specifically like this).
As I understand it, your only safe options are to install it to another partition or to an external drive, SSD recommended. APFS drives of 256 GB or less cannot be partitioned as that is the minimum size of an APFS partition. I discovered this when attempting to partition my 256 GB MacBook Pro. I’m running the latest OS version, 10.15.6.